1. Field of the Invention
The present invention relates to a resonant power source apparatus that is highly efficient, low noise, and inexpensive.
2. Description of the Related Art
FIG. 1 shows a current-resonant power source apparatus according to a related art. In FIG. 1, ends of a DC power source 1 are connected to a series circuit consisting of a MOSFET as a switching element QH and a MOSFET as a switching element QL. An end of the switching element QH is connected to a positive electrode of the DC power source 1. An end of the switching element QL is connected to a negative electrode of the DC power source 1.
The switching element QH is connected in parallel with a diode D2. The switching element QL is connected in parallel with a diode D1. The switching element QH is connected in parallel with a voltage-resonant capacitor Crv.
The voltage-resonant capacitor Crv is connected in parallel with a current-resonant circuit consisting of a current-resonant capacitor Cri, a resonant reactor Lr, and a primary winding P1 of a transformer T. The voltage-resonant capacitor Crv, current-resonant capacitor Cri, resonant reactor Lr, and the primary winding P1 of the transformer T form a resonant circuit.
The resonant reactor Lr may be a leakage inductance of the transformer T. The voltage-resonant capacitor Crv may be a parasitic capacitance of the switching element QH. The diode D2 connected in parallel with the switching element QH may be a parasitic diode of the switching element QH. The diode D1 connected in parallel with the switching element QL may be a parasitic diode of the switching element QL.
The primary winding P1 and a secondary winding S of the transformer T are wound so as to generate in-phase voltages. The secondary winding S of the transformer T is connected to a rectifying/smoothing circuit consisting of a rectifier D0 and a smoothing capacitor Co. The rectifying/smoothing circuit rectifies and smoothes a voltage (on/off-controlled pulse voltage) induced by the secondary winding S of the transformer T, to provide a DC output for a load 4.
Both ends of the smoothing capacitor Co are connected to an output voltage detector 5. The output voltage detector 5 detects an output voltage of the smoothing capacitor Co, generates an error voltage signal indicative of a difference between the detected voltage and a reference voltage, and supplies the error voltage signal through a photocoupler PC to a pulse ratio controller (PRC) 2. Based on the error voltage signal, the pulse ratio controller 2 generates a pulse ratio control signal to control the ON duty ratio of a pulse. The pulse ratio control signal is used by a high-side driver 3 to alternately turn on/off the switching elements QH and QL in such a way as to maintain a constant voltage for the load 4. To alternately turn on/off the switching elements QH and QL, voltage is applied to gates of the switching elements QH and QL.
Operation of the resonant power source apparatus of the above-mentioned configuration will be explained with reference to a timing chart of FIG. 2.
The ON period of the switching element QL is set to a given time width that involves a proper margin with respect to a half period of the resonance frequency of the resonant circuit consisting of the current-resonant capacitor Cri and resonant reactor Lr. The ON period of the switching element QH is determined according to the error voltage signal from the output voltage detector 5.
In FIG. 2, VQL is a terminal voltage of the switching element QL, IQL is a current passing through the switching element QL, IP1 is a current passing through the primary winding P1, VS is a terminal voltage of the secondary winding S, and ID0 is a current passing through the rectifier D0. The resonant reactor Lr is sufficiently smaller than the exciting inductance Lp of the primary winding P1, and the voltage-resonant capacitor Crv is sufficiently smaller than the current-resonant capacitor Cri.
In a period T1, the switching element QH is OFF, and the switching element QL changes from ON to OFF. Energy accumulated in the resonant reactor Lr and the exciting inductance Lp of the transformer T produces the resonant current IP1 that is passed from the resonant reactor Lr and exciting inductance Lp to the voltage-resonant capacitor Crv, current-resonant capacitor Cri, and primary winding P1. The exciting inductance Lp of the transformer T, resonant reactor Lr, and voltage-resonant capacitor Crv resonate to discharge the voltage-resonant capacitor Crv. As a result, a voltage VQH decreases and the voltage VQL increases.
In a period T2, the switching elements QL and QH are OFF. The voltage-resonant capacitor Crv completes the discharging, the voltage VQH is zero, and the voltage VQL is equal to a terminal voltage of the DC power source 1. The resonant current IP1 continuously passes in a clockwise direction through a path passing along the resonant reactor Lr, diode D2, current-resonant capacitor Cri, and primary winding P1. When the switching element QH is turned on, a period T3 starts. In the period T3, the switching element QH is ON and the switching element QL is OFF. The current IP1 decreases and continuously passes in a clockwise direction through a path passing along the resonant reactor Lr, switching element QH, current-resonant capacitor Cri, and primary winding P1. When this current becomes zero, a period T4 starts.
In the period T4, the switching element QH is ON and the switching element QL is OFF. The direction of the current IP1 is reversed. Namely, the resonant current IP1 passes in a counterclockwise direction through the primary winding P1 current-resonant capacitor Cri, switching element QH, and resonant reactor Lr, to reset the magnetic flux of the transformer T.
In the periods T1 to T4, the exciting inductance Lp of the transformer, the resonant reactor Lr, and the current-resonant capacitor Cri resonate to pass the current IP1.
In a period T5, the switching element QH turns off and the switching element QL is OFF. The current IP1 of the primary winding P1 of the transformer T is a resonant current that passes in a counterclockwise direction through the path passing along the primary winding P1, current-resonant capacitor Cri, voltage-resonant capacitor Crv, reactor Lr, and primary winding P1. The exciting inductance Lp of the transformer T, the reactor Lr, and the voltage-resonant capacitor Crv resonate to charge the voltage-resonant capacitor Crv, increase the voltage VQH, and decrease the voltage VQL.
In a period T6, the switching elements QH and QL are OFF. The voltage-resonant capacitor Crv is charged to the voltage of the DC power source 1, the voltage VQH becomes equal to the voltage of the DC power source 1, and the voltage VQL becomes zero. As a result, the resonant current IP1 continuously passes in a counterclockwise direction through the primary winding P1, current-resonant capacitor Cri, DC power source 1, diode D1, and reactor Lr.
In a period T7, the switching element QL turns on and the switching element QH is OFF. The resonant current IP1 continuously passes in a counterclockwise direction through the path passing along the primary winding P1, current-resonant capacitor Cri, DC power source 1, switching element QL, and resonant reactor Lr. In the periods T5 to T7, the exciting inductance Lp of the transformer T, the resonant reactor Lr, and the current-resonant capacitor Cri resonate to pass the current IP1 (the current IQL of the switching element QL).
In a period T8, the switching element QL is ON and the switching element QH is OFF. The current IP1 continuously passes in the counterclockwise direction through a path passing along the primary winding P1, current-resonant capacitor Cri, DC power source 1, switching element QL, and resonant reactor Lr. The current ID0 starts to pass through the rectifier D0 on the secondary side.
In a period T9, the switching element QL is ON and the switching element QH is OFF. The current IP1 from the DC power source 1 passes through a path passing along the current-resonant capacitor Cri, primary winding P1, and switching element QL. The current ID0 continuously passes through the rectifier D0 on the secondary side.
In the periods T8 and T9, the resonant reactor Lr and current-resonant capacitor Cri resonate to pass the current IP1 (equal to the current IQL of the switching element QL). In the periods T8 and T9, energy is transmitted from the primary winding P1 to the secondary winding S of the transformer T. At this time, the current ID0 passed from the primary side to the secondary side increases along a curve as time passes. At a certain point, the current ID0 starts to decrease and becomes zero after a resonant period t1 (corresponding to the periods T6 to T9). The energy transferred to the secondary side is rectified and smoothed through the rectifier D0 and smoothing capacitor Co, so that DC power is supplied to the load 4.
In a period T10, the switching element QL is ON and the switching element QH is OFF. The current IP1 (equal to the current IQL of the switching element QL) provided by the DC power source 1 passes through the path passing through the current-resonant capacitor Cri, primary winding P1, resonant reactor Lr, and switching element QL. The current ID0 stops. In the period T10, the exciting inductance Lp of the transformer T, the resonant reactor Lr, and the current-resonant capacitor Cri resonate to pass the current IP1 (equal to the current IQL of the switching element QL). After the period T10 (corresponding to a period t2 determined by an oscillation frequency or a duty ratio), the switching element QL turns off and the switching element QH turns on to start a reset period t3 (corresponding to the periods T1 to T5).
If the switching element QL turns off in the period t1 for transferring energy to the secondary side, no zero-current switching is carried out, and therefore, the rectifier D0 causes recovery noise to increase the switching noise and switching loss of the switching element QL.
To avoid this problem, the ON period of the switching element QL must have a sufficient margin to cover a variation in the period t1 due to variations of the current-resonant capacitor Cri and resonant reactor Lr. Namely, the ON period of t1+t2 must be sufficiently long.
If the period t2 is too long, an effective value of the current IP1 increases to increase a loss. In addition, a period for applying voltage to the primary side of the transformer T is extended to increase a loss in the transformer T. The loss is irrelevant to an original purpose of supplying energy to the secondary side, and therefore, is undesirable. In addition, the loss deteriorates the efficacy of the power source apparatus.
To solve the problem, Japanese Examined Patent Application Publication No. JP7-63216 (Japanese Patent No. 2042563) discloses a series resonant converter. FIG. 3 shows the converter. The converter includes a series circuit of two switching elements 102 and 103, a series circuit of two rectifiers 104 and 105, and a series circuit of two capacitors 106 and 107. These series circuits are connected to a DC power source 101. A node between the rectifiers 104 and 105 is connected to a node between the capacitors 106 and 107. The node between the rectifiers 104 and 105 and a node between the switching elements 102 and 103 are connected to a series circuit of a primary winding of a transformer 108 and a reactor 109. A secondary winding of the transformer 108 is connected to a rectifier 110 and an output capacitor 111. A transformer voltage detector 130 detects a voltage on the secondary winding of the transformer 108. A logic circuit 150 compares the output of the transformer voltage detector 130 with drive signals for controlling the switching elements, turns on the switching elements 102 and 103 during a period in which the transformer 108 generates voltage, and turns off the switching elements 102 and 103 after a disappearance of the voltage of the transformer 108.
This series resonant converter zeroes voltage applied to the transformer when a period for supplying energy to the secondary side ends, thereby zeroing voltage induced by the secondary winding.